1. Field of the Invention
The present invention relates to a depolymerization process for recovering the original monomers from which polymerized products such as scrap or waste tires, rubber and plastics were created. Although reference is made in this application primarily to scrap tires and the recovery of monomers therefrom, it should be understood that a number of different polymerized products can substitute for the scrap tires referenced herein.
Each year approximately 240 million used tires are discarded. The vast majority of these used tires are put into landfills or, all too often, are disposed of illegally. Such disposal of scrap tires in landfills is becoming increasingly undesirable as significant environmental and social problems are associated with such activity. The availability of landfill space is diminishing, and there has been a national trend toward more stringent regulatory requirements on landfills. Hence, a considerable amount of attention has been devoted recently toward finding alternative uses for and methods of reclaiming scrap tires.
Some research and commercial efforts have been directed toward the development of new uses for old tires. Unprocessed used tires have been used in playgrounds, flower planters, and shoe soles, and tire pieces have been used as gaskets, shims, dock bumpers and shock absorbers. The use of processed used tires in road construction is also gaining some attention. Paving materials made from a combination of crumb rubber and asphalt may last up to three times as long as standard paving materials. However, because of the high costs associated with the use of scrap tires for roads, this approach has not gained wide acceptance. Studies continue to explore this use of scrap tires in addition to the somewhat more conventional use of tires as crash barriers and impact absorbers around highway and bridge abutments. However, these new uses for used tires only consume a minor portion of the annual accumulation of scrap tires.
Consequently, significant research and commercial activity has been directed toward development of the use of scrap tires as an energy source. The 240 million used tires discarded each year represent upwards of 7.times.10.sup.13 BTU's of energy. Two approaches to tapping this energy source have received most of the attention. In the first, either whole or shredded tires alone are burned for fuel in incinerators or specially designed boilers, in many cases to generate electricity, or are added directly to cement kilns. In many tire incinerators combustion is not complete, resulting in the discharge of smoke and objectionable odors. Meeting federal emissions regulations for any combustion system is costly. In the second approach, tires have the steel belts removed by extensive shredding and are then combined with wood, paper pulp, or other refuse to get a fuel blend that can be burned to provide energy. While the combination of tires with other materials results in a higher total average BTU content for burning as compared to the other materials alone, difficulty in handling, which typically includes special feeding and blending equipment, makes this second approach a rather unattractive method for reclaiming scrap tires. Moreover, for some the incineration of waste plastics has been used to recover their BTU value, identical to the use of scrap tires as fuel.
While such approaches might seem attractive given rising natural-gas and fuel-oil costs, one major drawback to the approach is that valuable basic chemical building blocks or monomers, such as styrene, instead of being recovered, are consumed. The cost of these destroyed or lost monomers includes the costs, in energy and finite natural hydrocarbon resources, of exploration and drilling for new oil and the costs of transporting the crude oil and converting it to the intermediate chemicals from which rubber is made. Ethylene, propylene, butadiene, and styrene are a few examples of monomers derived from petrochemical sources and used in tire manufacture. The total energy required to make the monomers in the tire is on the order of 60,000 BTU per lb. The fuel energy value of a tire is approximately 15,000 BTU per lb. The cost to the environment of using the valuable monomers as fuel, rather than reclaiming and recycling them, would include the costs of the energy and finite natural resources used to make them, which are permanently lost, versus the relatively meager amount of energy and no natural resources recovered when they consumed. In addition, the costs to the environment of replacing rather than reclaiming the monomers includes the burden of the additional carbon dioxide generated by the energy used in replacement. Carbon dioxide, according to many studies, contributes to global warming. Thus, given the drawbacks associated with these uses for scrap tires, there has been a search for alternative uses for scrap tires that are less costly and that have minimal adverse impact on the environment.
Tires generally consist of rubber, carbon black, steel, fabric and other additives. Styrene-butadiene rubber is most commonly used in tire manufacturing, usually in combination with other elastomers such as natural rubber and ethylene propylene diene monomer (EPDM). Carbon black is used in the manufacture of tires to strengthen the rubber and increase resistance to abrasion. Steel, fiberglass, or fabric in the form of cords or belts is also present for reinforcement in the majority of tires produced today. Finally, other additives, such as antioxidants and antiozonants, are used in the tire manufacturing process to inhibit rubber deterioration and slow aging.
Polymerization is the process in which individual monomers join together in large numbers to form a polymer molecule. Where two different monomers join to form a polymer chain, a copolymer is produced. There are two broad classes of polymers and copolymers based on their polymerization: Condensation polymers, such as polyesters, nylon, polycarbonates, and polyurethanes, are those whose polymerized form has a lower molecular weight than the sum of the monomers used to make it (the balance is generated as other chemicals such as methanol or glycols during polymerization). Addition, or chain-growth, polymers, such as polyethylene and polypropylene, are those whose polymerized form has the same molecular weight as the sum of the monomers used to make them. Addition, or chain-growth, polymers are made in specific conditions of temperature and pressure and in the presence of an initiator (a form of catalyst) in which the polymer chain is propagated, or `zipped` together. Styrenebutadiene rubber, EPDM, and natural rubber, which are the polymers used in tire manufacture, are additive polymers.
There are basically two ways to break down a polymer: pyrolysis and depolymerization. Pyrolysis, also known as thermal cracking, is a process in which polymer molecules are heated until they fragment into several smaller, dissimilar, random-sized molecules. Pyrolysis typically results in the polymer molecules breaking down into a complex mixture of alcohols, hydrocarbons, and other molecules, none of which is an original monomer. Overall, the thermal conditions required for depolymerization are significantly milder than those associated with pyrolysis processes.
Depolymerization, the second way to break down a polymer, is essentially the opposite of polymerization. In the depolymerization of condensation polymers, prior art teaches several hydrolytic methods, such as glycolysis, methanolysis or hydrolysis, categorized by the depolymerization reactant used, such as glycol, methanol or water, respectively, wherein, under specific conditions of temperature and pressure and, sometimes, in the presence of a catalyst, the reactant is added to the polymer causing the polymer chain to separate into its original monomers. An example of these methods is the recycling of PET (polyethylene terephthalate) bottles by a methanolysis process which produces the raw material DMT (dimethyl terephthalate, a precursor to PET), and ethylene glycol. The DMT is then blended with virgin feedstock and FDA-acceptable polymers for food bottles are made. Hydrolytic depolymerization methods have not proven to be effective with addition polymers.
2. The Prior Art
While not limited solely to addition, or chain-growth, polymers, the present invention teaches a method for their depolymerization. The process specifically creates conditions of temperature and pressure and the presence of a catalyst to depropagate or depolymerize these polymers to their constituent monomers. The depolymerization temperature and pressure ranges for many types of polymers and copolymers of the monomers from which the tires are made are well documented in the technical literature. The thermodynamics for the depolymerization of polymers is elucidated in "Thermodynamics of Polymerization" by H. Sawada, published by M. Dekker, 1976. As explained by Sawada, each polymer will have different conditions for depolymerization. For example, polybutadiene depolymerizes in the 325.degree. C. to 475.degree. C. range, while a 75/25 polybutadiene/styrene copolymer depolymerizes in the 327.degree. C. to 430.degree. C. range. Generally, the temperatures involved in the depolymerization of the polymers and copolymers from which tires are made are in the 135.degree. C. to 500.degree. C. range. However, neither the technical literature nor prior patents teach a process for the depolymerization of additive polymers on a commercial basis.
Tires contain a number of different polymers blended together. Waste plastics from post-consumer wastes are generally mixed polymers unless hand or mechanical means of separating the various polymers are used. The present invention teaches a method of depolymerizing and recovering the monomers from each polymer in a mixture of polymers by a succession of reactors operating at different conditions of temperature and pressure. Prior art does not reveal a method for the sequential depolymerization of polymers in a single process.
The stress cracking of rubber by ozone has been known for decades. Ball and Youmans reported in Rubber Age 55, 481-3, 1944, that sheets of carbon black vulcanizates of tire tread cracked in 30 minutes in ozonized air at temperatures of 25 deg F., thus demonstrating that treatment with ozone gas will cause cleavage of vulcanized rubber molecular linkages. Baker and King in 1950 and Allison and Stanley in 1952 reported that stressed natural and synthetic rubbers relax when exposed to ozonized air. The inclusion of antioxidants and antiozonate chemicals in rubbers and polymers has been and is being practiced to suppress the deleterious effects of ozone and is well documented in prior art. The present invention teaches the use of ozone treatment to break down the cross-linked rubber making it more susceptible to thermal depolymerization thus using an otherwise `undesirable` reaction to its advantage. While the deleterious effect of ozone on rubber and other polymers has been known for a considerable time, there is no readily available information that past investigators utilized ozone to beneficially enhance a depolymerization reaction process.
The cryogenic grinding of tires and other materials is taught in the prior art. Lovette, Jr. teaches in U.S. Pat. No. 4,025,990 a process for reclaiming rubber, metal and fabric from whole tires by cooling the tires in a cryogenic freezer to the embrittlement temperature, comminuting them in a comminution device and passing the resulting particles through a series of screening and density classification operations followed by magnetic material separation and further cryogenic size reduction to produce rubber crumb having a particle size of about 1/20 inch or less. Other inventors have proposed similar procedures to achieve particle size reduction. The cryogenic method illustrated in the present invention is a preferred embodiment of achieving reduced particle size but other methods may be used within the spirit and scope of the invention.